Air travel in Europe inched back to normal Wednesday, as officials estimated that newly opened flight routes would permit air traffic to approach 75 percent of its normal capacity. Ash plumes from Iceland's Eyjafjallajökull volcano had all but extinguished flight operations across the U.K. and mainland Europe for the better part of a week.

Barring a tragic outcome, which is thought to be unlikely, it will be difficult to know the extent to which jet engines can tolerate mild to moderate intakes of ash. The damage might be cumulative and is tough to detect, says Michael Fabian, a professor of mechanical engineering at Embry-Riddle Aeronautical University in Prescott, Ariz.

In an effort to keep planes and passengers safe, officials broke the affected areas of airspace around Europe into three tiers: normal flight zones where ash no longer poses a risk, no-fly zones where ash remains in high concentrations, and intermediate, potentially hazardous zones where flights can proceed with caution, subject to route restrictions and other limitations. To draw those boundaries, flight controllers were forced to determine what constitutes an acceptable level of volcanic ash, despite a lack of data to inform their assessment.

The three-tiered approach is "designed to prevent airliner contact with levels of ash that are 10 times the normal levels," says Kyla Evans, a spokesperson for the Brussels-based intergovernmental air traffic agency EUROCONTROL. "In other areas, where there is possible ash contamination, but at lower concentrations, aircraft can fly."

Fabian says that jet engines' tolerance to ash is not well defined. "That's an unknown—that's the problem," Fabian says. "There really isn't much data about how much can you tolerate. It's going to vary tremendously by engine."

Volcanic ash is highly abrasive and can melt down to a glass in the extreme heat of a turbine engine; past encounters with volcanic clouds have produced near-catastrophic results. In 1989, for instance, a KLM airliner approaching Anchorage, Alaska, passed through an ash plume spewing from nearby Mount Redoubt. All four engines flamed out, and the plane glided to 4,000 meters in altitude before the pilots were able to restart them and land safely. According to a U.S. Geological Survey report on volcanic aviation hazards, the plane suffered $80 million in damage, including the cost of a new set of engines. So air operators pay close attention to volcanic ash, for both safety and economic reasons. "You know it's going to be bad on the engine, so you historically just avoid it," Fabian says.

If one of the flights now passing through European airspace suffers any detrimental effects from minor exposure to volcanic ash, those effects may not present themselves as suddenly as in the 1989 incident over Anchorage. "It's a cumulative thing," Fabian says. "Who knows if it happens on the first flight?"

Evans of EUROCONTROL adds that flights passing through potentially contaminated areas may be required to report back with the results of postlanding inspections, but Fabian says that such checkups do not reveal all potential damage. "I'm sure they'll borescope after flights—that's like a video camera with a snake—and they'll run it up the tail and look at the blades and the surface," he says. And indeed, that is precisely what engine manufacturer Pratt & Whitney advised its customers to do in the event of ash exposure. "But the trouble is they're not seeing the inside of the blades, and you won't get that until you do an overhaul or a flow test of the blades." Pulling the engine apart for that kind of testing simply is not possible for operators with a manifest of prescheduled flights, Fabian adds.

"I'm sure they're using their basic instrumentation to see if the temps look right in terms of thrust level and pressure levels, but there's a question mark out there in terms of how the turbines are surviving," he says. "It's going to be interesting to see how things play out. You hope that they don't push it too hard."

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